IGCSE Chemistry 0620: Testing Stream Water for Carbonate and Nitrate Ions

A stream-monitoring volunteer group takes a water sample from a stream that runs past a farm. They suspect the sample may contain both carbonate ions, washed from an old lime-spreading track nearby, and nitrate ions, from fertiliser run-off. The group splits the sample into two portions to test for each ion in turn.

(a) Describe the test, including the reagent(s) added and the final observation, that would confirm carbonate ions are present in the first portion. [2]

(b) Describe the test, including the reagent(s) added and the final observation, that would confirm nitrate ions are present in the second portion. [3]

(c) Both tests in (a) and (b) work by detecting a specific gas, rather than by looking directly at the original water sample. Name the gas produced in each test. [2]

Part (a): Testing for carbonate ions

Add dilute acid to the first portion. If carbonate ions, $\text{CO}_3^{2-}$ , are present, effervescence (bubbling) occurs as carbon dioxide is released:

$\text{CO}_3^{2-}(aq) + 2\text{H}^+(aq) \rightarrow \text{H}_2\text{O}(l) + \text{CO}_2(g)$

To confirm the identity of the gas, it is passed into limewater, which turns milky. This confirms both that a gas was produced and that the gas is carbon dioxide, so carbonate ions are present in the sample.

Part (b): Testing for nitrate ions

Add aqueous sodium hydroxide and a small piece of aluminium foil to the second portion, then warm the mixture carefully. If nitrate ions, $\text{NO}_3^-$ , are present, the aluminium reduces them under these alkaline, warmed conditions to produce ammonia gas.

The ammonia gas is confirmed using damp red litmus paper held near the mouth of the tube: the paper turns blue, confirming ammonia gas and therefore that nitrate ions are present in the sample.

Part (c): Identifying the gas in each test

Both tests do not look at the original ions directly, they rely on producing and identifying a specific gas:

(a) The gas produced (and tested with limewater) is carbon dioxide, $\text{CO}_2$ .
(b) The gas produced (and tested with damp red litmus paper) is ammonia, $\text{NH}_3$ .

Final answers

(a) Add dilute acid; effervescence, and the gas turns limewater milky $\rightarrow$ carbonate ions present.
(b) Add aqueous sodium hydroxide and aluminium foil, warm carefully; ammonia gas turns damp red litmus paper blue $\rightarrow$ nitrate ions present.
(c) (a) $\boxed{\text{carbon dioxide}}$ ; (b) $\boxed{\text{ammonia}}$ .